The present invention relates generally to the field of laser diodes, and more particularly to architecture for short-wavelength nitride based laser diode arrays.
Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as optical storage.
Laser diode arrays are desirable for application to high speed laser printing. Printing at high speeds and at high resolution requires laser arrays due to the fundamental limits of polygon rotation speed, laser turn-on times and laser power. Laser diode arrays have previously been employed using red and infrared laser diode structures. Dual-spot red lasers and quad-spot infrared lasers have been used for laser printers.
Laser diodes based on higher bandgap semiconductor alloys such as AlGaInN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, A. Kuramata et al., xe2x80x9cRoom-temperature CW operation of InGaN Laser Diodes with a Vertical Conducting Structure on SiC Substratexe2x80x9d, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998), S. Nakamura et al., xe2x80x9cCW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substratesxe2x80x9d, Applied Physics Letters, Vol. 72(6), 2014 (1998) and S. Nakamura and G. Fasol, xe2x80x9cThe Blue Laser Diode-GaN based Light Emitters and Lasersxe2x80x9d, (Springer-Verdag, 1997) all of which are incorporated by reference in their entirety.
Extension of quad-spot laser diodes to shorter wavelengths enables printing at higher resolution. Growth of quad-spot laser diodes on conducting substrates allows a common backside contact for all laser diodes in the array.
Architectures using conducting substrates for nitride dual-spot laser diode arrays allow the use of common backside contacts for all devices in the laser diode array to permit a compact layout. Elimination of cleavage plane misalignment with GaN by using a conducting substrate enables the formation of high quality cleaved mirror facets without dry etching.
The metallization scheme on the frontside of the nitride based laser diode array structures is similar to that used in red and infrared lasers. Frontside contacts on quad-spot laser array structures are arranged to provide separate p-metal contacts for each laser diode. However, certain compact layouts result in small unpumped sections in two of the lasers making up the quad-spot structure due to necessary breaks in the contact pad structure to allow contact paths to the remaining laser pair. The presence of unpumped sections in a nitride based laser diode can have adverse effects such as raising the required threshold current density.
A layout that overcomes the problem of unpumped sections but still retains the compact structure needed in printing applications employs air-bridge contact structures to cross over intervening metal contact areas. This isolates the contacts to the laser diodes while avoiding having unpumped sections. Air-bridge contact structures also allow minimization of parasitic capacitance effects between contacts.
Surface emitting quad-spot laser diode arrays may be made on a conducting substrate using a channel structure between two dual-spot laser diode arrays. The channel structure contains mirrors to outcouple light at various angles to the laser cavity. This scheme is readily generalizable to produce surface emitting laser diode arrays containing an arbitrary number of laser diodes.
Thus, the present invention and its various embodiments provide numerous advantages as will be described in further detail below.